In the past, machines that were used to inspect pipes used electromagnetic waves to detect transverse and longitudinal faults. Also combined as a feature in such machines was the use of gamma rays to detect wall thickness. Pipes were fed into these machines and rotated as they were being fed. However, the downside of this type of inspection is that it missed about 70 percent of the pipe wall area. Furthermore, the wall thickness test involved an averaging of a measurement of two opposing walls; therefore, a complete picture of the wall thickness at the point of inspection was not clearly known.
More recently, ultrasonic testing machines have been developed. Some of these use a plurality of ultrasonic sensors which are fixedly mounted while the pipe is rotated and advanced over the sensors. The sensors provide information on the pipe condition, including defects as well as wall thickness. The problem with such machines is that they require so many sensors and complex computer equipment to analyze all of the signals obtained from the sensors on a real-time basis, that inspection using such machines proves to be a very costly endeavor. Using the stationary multiple sensors as just described results in a charge to the customer of approximately 4-6 times as much as pipe inspected using the older technology involving electromagnetic waves.
There have also been machines that use ultrasonic technology where the sensors themselves spin around the pipe. Typically, these sensors generate a high-frequency, low-voltage signal which must go through a slip ring because the sensors themselves are rotated around the pipe. The signal, after going through the slip ring, goes to a processor which converts the information to a useful form with regard to imperfections in the pipe. The problems associated with machines of this design is that the high-frequency, low-voltage signal going through the slip ring was subject to interference which resulted in affecting the signal accuracy transmitted from the sensor to the processor.
The apparatus of the present invention exhibits a marked improvement from known ultrasonic sensors in that the processor rotates about the pipe with the sensors, and the signal going through the slip ring is the output from the processor. Therefore, the signal generated by the processor which is low-frequency and high-voltage is not as easily subject to interference as the high-frequency, low-voltage signals put through the slip rings in the prior designs.
Another problem exhibited by ultrasonic testing machines is the need to get good contact at the surface to obtain the readings. The apparatus of the present invention has incorporated a sprayer/roller combination which in effect paints a liquid surface on the pipe as it advances toward the sensors. The sprayers in combination with the rollers spread a film over the outer surface of the pipe to facilitate getting a good contact between the sensor holder and the pipe so that accurate readings can be obtained. This feature, in combination with a floating shoe for the sensor, improves accuracy due to better contact with the pipe wall.
Another feature of the apparatus of the present invention is its compactness which allows it to be added to an existing testing facility which employs a combination of electromagnetic and gamma rays to test pipe. Thus, what results is a combination of machines that when put together allow tests to be done economically yet greatly improve the coverage problems associated with using the electromagnetic waves/gamma ray technology to test pipe. By controlling the advance speed of the pipe, as well as the rotational speed of the sensors, coverage over 100 percent of the pipe area can be assured.
Of interest as far as the state of the prior art are U.S. Pat. Nos. 5,007,291; 4,562,738; 4,843,884; 4,328,708; 4,596,953; and 4,672,852; all of which illustrate ultrasonic testing machines with rotating sensors in combination with the use of slip rings and an applied liquid. Several ultrasonic testers are portable for use on existing pipes, such as U.S. Pat. Nos. 4,331,034 and 4,531,413 and 4,586,379. Other testers employ stationary probes such as U.S. Pat. Nos. 4,718,277; 4,660,419; 4,567,747; and 4,475,399.